Triethylphosphate (TEPO) is a liquid chemical commonly used as the phosphorous-containing source for the production of PSG (phosphosilicate glass) or BPSG (borophosphosilicate glass) in the microelectronics industry. PSG and BPSG are used in semiconductor devices as insulating layers that are deposited between metal or conducting layers. BPSG films are typically used as planarization films which can provide excellent step coverage for high density microelectronic devices. The presence of percent levels of boron and phosphorus in the BPSG films has the effect of decreasing the glass transition temperature relative to the silicate glass, thus decreasing the thermal budget of a fabrication process while providing gap-filling properties that enable the BPSG layer to meet the requirements of microelectronic manufacturing processes.
Manufacturers require phosphorous sources, such as TEPO, to be extremely pure with respect to certain metals such as arsenic and other metals. Certain metallic impurities may act as device poisons or have undesirable toxicological properties, especially in the case of arsenic, that may present significant environmental health and safety issues during the normal life cycle of the device, such as during manufacturing or waste disposal after use.
For the aforementioned reasons, manufacturers in the microelectronics industry require that TEPO meets very stringent specifications with respect to the concentration of certain metallic impurities. Typical concentration specifications for TEPO require that many alkali, alkaline earth, rare earth, transition and main group metals are present at less than 5 ppb, or more commonly less than 1-3 ppb.
However, raw TEPO has arsenic levels of 50 ppb (parts per billion) to over 10,000 ppb depending on the particular source and how it was processed. Achieving arsenic levels of less than 5 ppb, or more commonly less than 1-3 ppb, presents a substantial challenge for arsenic in particular, which is a common impurity in phosphorus-containing materials.
The removal of trace levels (about 50 ppb to 10,000 ppb) of arsenic impurities from TEPO often cannot be accomplished by the use of common separation techniques because of the similarity in the chemical and physical properties of the phosphorus and arsenic compounds.
There are numerous examples in the prior art of the removal of arsenic-containing impurities from various systems, including gaseous streams such as acetylene or silane, liquid hydrocarbon fractions used in the petrochemical industry, aqueous systems, and phosphate based fluid fertilizer streams. The arsenic-containing components removed during processing are either tri-valent or penta-valent, and either organic and inorganic.
The majority of the previous works described the removal of arsenic such that the treated material had a final arsenic content of 1-1000 ppm (parts per million, 1 ppm=1000 ppb). The description of the removal of arsenic to ultra-trace levels (10 ppb or less) found in the prior art was for the preparation of high purity silane used for microelectronic applications. In the latter case the primary arsenic contaminant is gaseous arsine, as opposed to the present disclosure in which the arsenic impurities are believed to be either miscible liquids, such as triethylarsenate (TEASAT) or solubilized inorganic salts.